Nine rings to cope with ITER’s powerful magnetic fields

Every legend has a ring somewhere
in its narrative. Most of the times it is lost but when found it has the power
to awake us, liberate us or even make us invincible. ITER, embarking on an epic journey to bring
the energy of the sun to Earth, could be no exception. In fact, one ring will not suffice to “save”
its impressive magnets from the electromagnetic loads as they confine the
super-hot plasma. The biggest fusion device will require nine rings and Europe
is responsible for delivering them.

ITER will rely on a sophisticated
system of superconducting magnets consisting of the central solenoid, which can
be described as its backbone; the correction coils, which will reduce the range
of magnetic errors created by imperfections due to the location and geometry of
other coils; six Poloidal Field coils responsible for the shape and stability of
the plasma, and 18 Toroidal Field (TF) coils which will create a magnetic cage
to entrap the hot gas. To cope with the fatigue exercised on the TFs, and with
the deformation resulting from the powerful magnetic fields, three
pre-compression rings will be placed on top of them and three below them. An
extra set of three will be manufactured as spare in case there is a need in
future to replace the lower set. The fiberglass composite rings, consisting of
more than a billion miniscule glass fibers, will be glued together by a high
performances epoxy resin. They will have a diameter of approximately 5 m, a
cross-section of nearly 300 mm x 300 mm and will weigh slightly more than 3 T.

We visited the workshop of Airbus
Defence and Space (Airbus D&S) in Madrid where teams of engineers and
technicians are aiming to complete a full-scale prototype so that “real”
production of pre-compression rings can kick-off in future. The components are
manufactured using a well-established technology in the field of aerospace known
as automated fiber placement. To reach this stage, three full-scale ring slices
of fiberglass epoxy resin have been cured. Previously, one additional
full-scale slice has been produced and has undergone the first stage of the
qualification phase. F4E’s experts will soon examine the results of the
full-scale prototype in detail before production advances.

Several kilometres away, another
team of technicians is working at CNIM’s workshop in Toulon (FR) trying to
develop the three spare pre-compression rings. They have been building on the work already performed by companies such
as Exel (FI), Solvay (UK), CMF (IT), CMC (FR) and test laboratories such as
Rescoll (FR), Etim (FR) and KIT (DE). As standard practice requires, they have
started with the fabrication of a mock-up, which is roughly 1005 mm in
diameter, almost 1/5 of the real size of the component. The material they have
opted to use is pultruded laminate, which will be wound along the trajectory of
the ring. As the tooling is slowly bending the material, a bonding tape is
applied on its layer. This helical movement is repeated several times until the
pre-compression ring mock-up consists of multiple layers which will then be
cured. Afterwards, the mock-up is placed on an equipment to be machined to the
final dimensions and to remove excessive material.

Next, it will go through
nondestructive testing (NDT) and in the end it will be transported to ENEA’s
facility where final destructive tests will be performed.What is the state of play now? A series of
mock-ups has been produced and qualification tests are being performed. It is
estimated that by next year the “real” production of the spare pre-compression
rings will begin.